Due to different error sources (e.g., manufacturing tolerance, misalignments, etc.), a machine tool head generally approaches or moves to a position that deviates from the theoretical (ideal) position according to the CNC program or control. In this way, displacement or positioning errors occur. The underlying errors are sometimes classified into static and dynamic positioning errors, wherein static positioning errors result from machine geometry, structure rigidity, etc., whereas dynamic errors are based on servo parameters, acceleration, etc. In order to correct or compensate for positioning or displacement errors, firstly, it is necessary to measure or determine these errors. The prior art provides different methods for measuring errors with laser trackers or interferometer telescoping ball bars.
Depending on the geometry and/or size of the CNC machine, however, these conventional methods are not always suitable. For example, in case of a large machine, at different positions, the reflector being mounted to the machine tool head will be too far from the laser source so that no reliable position determination may be made. In view of this, a need exists for an improved method and system for determining positioning accuracy of a CNC machine, in particular, for the case of large machines.
The invention thus provides a method for checking positioning accuracy of a CNC machine with a machine tool head and a machine table, using an x, y, z coordinate system, wherein the z coordinate is the coordinate perpendicular to a machine table surface, the method comprising the steps: (a) arranging a plurality of vessels on the machine table, wherein the vessels are in fluid connection with each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with a liquid; (b) mounting a distance sensor to the machine tool head; (c) positioning the machine tool head to a position perpendicular above a surface of the liquid in one of the vessels; (d) approaching the surface of the liquid with the distance sensor and determining the z coordinate for which the distance sensor touches the surface of the liquid; or, for a predetermined z coordinate of the machine tool head, determining the distance between the distance sensor and the surface of the liquid; wherein steps (c) and (d) are repeated for each vessel to determine a respective z coordinate of the surface of the liquid; an apparatus for checking positioning accuracy of a CNC machine, the apparatus comprising a plurality of vessels in fluid connection to each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with a liquid; and/or a system for checking positioning accuracy comprising: a CNC machine with a machine tool head and a machine table, an apparatus for checking positioning accuracy of a CNC machine, the apparatus comprising a plurality of vessels in fluid connection to each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with a liquid, wherein the liquid is an electrolyte solution, and a vessel electrode is placed in contact with the electrolyte solution in each vessel, wherein a distance sensor is mounted to the machine tool head.
In particular, a method for determining a positioning error of a CNC machine is provided, the machine having a machine tool head and a machine table, using an x, y, z (Cartesian) coordinate system, wherein the z coordinate is the coordinate perpendicular to the machine table surface, the method comprising the steps of    a) arranging a plurality of vessels on the machine table, wherein the vessels are in fluid connection with each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with a liquid;    b) mounting a distance sensor to the machine tool head;    c) positioning the machine tool head to a position perpendicular above the surface of the liquid in one of the vessels;    d) approaching the surface of the liquid with the distance sensor and determining the z coordinate for which the distance sensor touches the surface of the liquid,
or,
for a predetermined z coordinate of the machine tool head, determining the distance between the distance sensor and the surface of the liquid;
wherein steps c) and d) are repeated for each vessel to determine a respective z coordinate of the surface of the liquid.
This method allows determining positioning or geometric accuracy in z direction for many different machine geometries and sizes. The machine tool head is moved to an x-y position corresponding to a position of one of the vessels. In this way, the sensor is positioned over the surface of the liquid in said vessel (at a height or z coordinate given by the CNC control), permitting a determination of the height or level of the liquid surface. This determination may be performed in a non-contact manner, e.g. by measuring the distance between the distance sensor or the machine tool head and the surface of the liquid. As an alternative, the liquid surface may be approached from above (i.e. along the z direction) until the sensor touches the liquid surface. As the arrangement of vessels forms a system of communicating vessels, the height or z coordinate of the surface of the liquid is the same throughout the vessels. Hence, the z coordinate determined by the CNC system, theoretically or ideally, should be the same for all vessels. As a consequence, any deviation or difference in the z coordinate from one vessel to another vessel allows for a determination of a positioning accuracy and, thus, a positioning error in z direction.
The “reference system” given by the communicating vessels having an equal level in all x-y positions permits to use the generally inaccurate CNC machine for measuring and determining its own and inherent errors. By repeating steps c) and d) for each vessel, a map of the z coordinates of the surface of the liquid at the different vessel positions may be obtained.
The CNC machine may be a gantry machine.
Different types of distance sensors are possible. For example, the distance sensor may be a non-contact distance sensor. In particular, it may be a laser distance sensor, a microwave distance sensor or a radar distance sensor. These sensors allow for a fast and accurate determination of the distance between the sensor and/or the machine tool head and the liquid surface, for example, via a time-of-flight measurement. For these cases, preferably, the liquid is reflective for the electromagnetic waves used. For example, when using a laser distance sensor, the liquid may be non-transparent for the laser light.
As an alternative, in step a), the liquid is an electrolyte solution and an electrode element, called vessel electrode, is placed in contact with the electrolyte solution in each vessel; in step b), the distance sensor comprises an electrode element, called tool head electrode, which is mounted to the machine tool head, wherein the material of the tool head electrode is different from the material of the vessel electrode; step c) comprises approaching the surface of the electrolyte solution in a vessel with the tool head electrode; and step d) comprises determining the z coordinate for which the tool head electrode touches the surface of the solution by or via determining an electrical potential difference between the tool head electrode and the respective vessel electrode.
In other words, the invention provides a method for determining a positioning error of a CNC machine, the machine having a machine tool head and a machine table, using an x, y, z (Cartesian) coordinate system, wherein the z coordinate is the coordinate perpendicular to the machine table surface, the method comprising the steps of    a) arranging a plurality of vessels on the machine table, wherein the vessels are in fluid connection with each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with an electrolyte solution, and wherein an electrode element, called vessel electrode, is placed in contact with the electrolyte solution in each vessel;    b) mounting an electrode element, called tool head electrode, to the machine tool head, wherein the material of the tool head electrode is different from the material of the vessel electrode;    c) approaching the surface of the electrolyte solution in a vessel with the tool head electrode;    d) determining the z coordinate for which the tool head electrode touches the solution surface by or via determining an electrical potential difference between the tool head electrode and the respective vessel electrode;
wherein steps c) and d) are repeated for each vessel to determine a respective z coordinate of the surface of the solution.
According to this alternative, the tool head electrode approaches the surface of the electrolyte solution within the open top vessel from above (i.e. along the z direction). Because of the use of different electrodes, i.e., electrodes of different materials, an electrical potential difference occurs as soon as the tool head electrode touches the surface of the electrolyte solution, similarly to the case of a galvanic cell. This enables the system to determine the z coordinate of the surface of the electrolyte solution to a high degree of accuracy.
The electrolyte solution may be a saline solution.
Each vessel electrode is positioned and/or arranged so that it is wetted by the electrolyte solution. For example, an electrode element, the vessel electrode, may be placed in each vessel. As an alternative, each vessel electrode may be embedded within a vessel wall (but in contact with the electrolyte solution) or may pierce the vessel wall from the outside of the vessel. As a further alternative, the vessel electrode may be formed as part of the vessel side wall or bottom wall, for example, as the base element of the vessel.
Steps c) and d) may comprise periodically and/or continuously determining the potential difference while approaching the surface of the solution until a predetermined potential difference threshold is exceeded. Such a regular and/or continuous check of the difference of the electrical potential enables a very precise determination of when the surface of the electrolyte solution is reached. Upon touching the surface of the solution with the vessel electrode, there will be a sudden increase in the electrical potential difference. By providing a predetermined threshold, noise during the measurement process may be reliably neglected.
In the methods described above, the approaching may be stopped as soon as, in step d), an increase in the potential difference, in particular above a predetermined potential difference threshold, is determined.
For the sake of simplicity, the “potential difference” may refer to the absolute value of the measured electrical potential difference, to avoid any complications because of the sign.
Such an immediate stopping reduces the time consumption and accelerates the overall method. The determination of the potential difference may involve a potential difference measurement device. The control of the CNC machine may periodically and/or continuously poll or sample the potential difference measurement device to determine whether an increase in the potential difference, particularly above a predetermined threshold, has occurred, followed by a stop signal to the machine tool head driving part. Alternatively, the potential difference measurement device may be configured to automatically send an indicator signal relating to the detected potential difference to the CNC machine. Such an indicator signal may be sent periodically or only if an increase in the potential difference, particularly above a predetermined threshold, is detected. The indicator signal may contain the value of the determined potential difference and/or (only) an indication that a predetermined threshold has been exceeded.
The approaching of the surface may be at a constant speed, in particular, as of a predetermined distance above the estimated level of the surface of the solution. For example, the speed may lie between about 2 mm/min to 20 mm/min.
The vessel electrode may be a metal electrode, e.g., made of aluminium. The tool head electrode may comprise a water-repellent material. At the side facing the surface of the solution, the tool head electrode may have a tip form; it may have a spherical and/or a conical shape. The tool head electrode may be a graphite electrode, a copper electrode or a silver electrode.
The use of a water-repellent or hydrophobic electrode, such as a graphite electrode, has the advantage that distortions or errors during the measurement (e.g., due to some capillary effect when approaching the surface of the solution and/or due to solution droplets adhering to the tool head electrode) are avoided or at least reduced.
Each vessel electrode and the tool head electrode may be electrically connected to a potential difference measurement device, for example, a voltmeter. Alternatively, each vessel electrode may be electrically connected to the machine table electrical ground.
Steps c) and d) of the above-described methods may be performed in an automated way, in particular, via a corresponding program of the CNC machine. For example, the x-y positions of the vessels as well as an estimated height or level of the surface of the liquid, particularly the electrolyte solution, and the movement path between the vessels may be programmed into the CNC machine. This allows for an efficient execution of the described methods for checking the positioning accuracy.
Step a) of the above-described methods may comprise:
mounting a light emitter to the machine tool head;
programming the x-y coordinates of the locations where to place the vessels into the CNC machine;
approaching each x-y coordinate with the machine tool head;
mounting a vessel at the location on the machine table identified by a light spot from the light emitter.
In this way, the locations where to mount the vessels are identified in a simple manner. Furthermore, the x-y coordinates for the vessel positions have to be programmed only once and, then, may be used both for mounting the vessels and for the subsequent step of positioning the machine tool head over the liquid surface in one of the vessels and/or approaching the surface of the electrolyte solution in the vessels, thus, leading to a highly efficient procedure. The light emitter may be a laser emitter.
In general, an arbitrary number of vessels may be used. Already two vessels allow determining a deviation or difference in the z coordinate between the locations of these tool vessels. Particularly when using more than two vessels, the vessels may be arranged in form of a one-dimensional or two-dimensional array. In particular, the vessels may be positioned on the lattice points of a lattice; for example, a rectangular or square lattice may be used.
In principle, the vessels may be fluidly connected in an arbitrary manner to form the system of communicating vessels. For example, a vessel may connect to each next neighbor. If the vessels are arranged on lattice points, each vessel may be fluidly connected to the vessels on neighboring lattice points. The fluid connection may achieved via tubes. The tubes may be flexible hoses or rigid pipes.
The described methods may further comprise the step of compensating a positioning error in z direction.
Such a compensation may be obtained in different ways. For example, the compensation may involve a mechanical rearrangement or realignment of the machine. Alternatively or additionally, a software compensation may be implemented in the control of the CNC machine. Different algorithms for software compensation are known. For example, a linear compensation resulting from a linear interpolation of the error or deviation values at neighboring vessel positions may be employed.
The invention further provides an apparatus for checking positioning accuracy of a CNC machine, the apparatus comprising a plurality of vessels in fluid connection to each other to form a system of communicating vessels, wherein the system of communicating vessels is filled with a liquid. The liquid may be an electrolyte solution, and an electrode element, called a vessel electrode, may be placed in contact with the electrolyte solution in each vessel.
This apparatus enables the performance of the above described methods.
The features mentioned above in connection with the method may also be provided in the context of the apparatus. For example, each vessel electrode may be placed within the vessel or may be part of the vessel; it may be a metal electrode and/or the vessels may be arranged in the form of an array.
The apparatus may comprise a support on which the vessels are mounted. With such a support, mounting and unmounting the apparatus, particularly the vessel arrangement, to a CNC machine is simplified and accelerated.
The invention also provides a system for checking positioning accuracy comprising:
a CNC machine with a machine tool head and a machine table;
an apparatus for checking positioning accuracy as described above;
wherein a distance sensor is mounted to the machine tool head.
The CNC Machine May be a Gantry Machine.
The system may further be configured to perform the above-described methods. In particular, the distance sensor comprises an electrode element, called tool head electrode, which is mounted to the machine tool head, wherein the material of the tool head electrode is different from the material of the vessel electrode, and the system may further comprise a potential difference measurement device configured to measure an electrical potential difference between a vessel electrode and the tool head electrode. The tool head electrode may be a graphite electrode.
In the system, the CNC machine may be programmed to position the machine tool head to a position perpendicular above the surface of the liquid in one of the vessels, and to determine the z coordinate for which the distance sensor touches the surface of the liquid, or, for a predetermined z coordinate of the machine tool head, to determine the distance between the distance sensor and the surface of the liquid, wherein the positioning and determining steps a repeated for each vessel to determine a respective z coordinate of the surface of the liquid.
In case of a using electrode elements, the CNC machine may be programmed to approach the surface of the electrolyte solution in a vessel with the tool head electrode and to determine the z coordinate for which the tool head electrode touches the surface of the solution by determining a potential difference between the tool head electrode and the respective vessel electrode,
wherein the approaching and determining steps are repeated for each vessel to determine a respective z coordinate of the surface of the solution.
Using the difference in the electric potential, the system allows for a fast and reliable check of the positioning accuracy of the CNC machine.
The CNC may also be programmed to compensate a positioning error in z direction.
Further aspects will be described in the following with reference to the accompanying figures.